A passive optical network (PON) is one system for providing network access over “the last mile.” The PON is a point to multi-point network comprised of an optical line terminal (OLT) at the central office, an optical distribution network (ODN), and a plurality of optical network units (ONUs) at the customer premises. Data is transmitted in the PON over optical fibers that connect the different PON components. When the physical conditions of the optical fibers deteriorate, the transmission of data is affected. For instance, breaks or partial breaks in the optical fibers can lead to a reduction in data transmission bandwidth, extensive traffic congestion, or a complete stop of data transmission.
Optical time domain reflectometry (OTDR) is one diagnostic tool that may be used to detect physical problems in fiber optics. In OTDR, a test signal comprising a light pulse is transmitted down the fiber, reflected at some point, and then measured using an OTDR receiver located at the same location from which the pulse originated. The delay time between transmission and reception of the pulse is used to calculate the round-trip distance of the pulse. The calculated round-trip distance is used to find the location of any deteriorations or breaks in the fiber, which may then be fixed or replaced.
One problem associated with OTDR is the separation of the test signal from the data signal. The test signal can be separated from the data signal using wavelength division multiplexing (WDM), which requires additional optical components in the PON. The test signal can also be separated from the data signal using frequency division multiplexing (FDM), in which case the technique is re-named optical frequency domain reflectometry (OFDR). Unfortunately, classical approaches to this require specialized transmitters and lead to reduced signal to noise ratios due to the reduced test signal power (e.g., 10% of the total transmitter power). A third technique is to use the user data signal as the test signal, such that the OFDR measurements are time-correlated with the data signal to recover the reflection delay times. However, such an approach may require significant amount of memory to process the two signals and may not be suitable for real-time implementation.